The present invention relates to a sputtering target having a mixture of oxides of indium, gallium, and zinc, and to a method for producing such a sputtering target.
Liquid crystal displays (LCDs) have long been indispensable in the electronics industry, and active-matrix displays have in particular enjoyed great popularity. For controlling, such active-matrix displays contain a matrix of thin-film transistors (TFTs), for whose manufacture it is necessary to produce thin layers of an active semiconductor. Amorphous silicon is conventionally used here as a semiconductor material. However, layers of amorphous silicon are characterized by relatively low charge carrier mobility.
For this reason, improved semiconductor materials are constantly being sought. In particular, materials from the group of the oxide semiconductors have turned out to be highly promising alternatives to amorphous silicon. Above all, mixed oxides of indium, gallium, and zinc exhibit properties that in many respects are improved compared to amorphous silicon. For example, layers of indium-gallium-zinc oxides have a very high charge carrier mobility of 10-50 Vs/m2, and are therefore better by orders of magnitude than the layers of amorphous silicon currently used in TFT-LCD displays. Moreover, indium-gallium-zinc oxides have a charge carrier concentration that is suitable for use in transistors. Their good semiconductor properties in connection with high transparency give these materials a number of attractive possible uses. For example, indium-gallium-zinc oxides can be used to produce thin-film transistors for LCD displays having a high switching frequency, or for organic light emitting diodes (OLEDs), which, as an emissive technology, require a current drive. In addition, the use of indium-gallium-zinc oxides enables an enlargement of the effective pixel surface, because the manufactured transistor is transparent.
The layers of the active semiconductor required for the production of thin-film transistors are conventionally applied by sputter deposition (sputtering). In sputtering, atoms or compounds are detached from a solid body, the sputtering target, through bombardment with energy-rich ions (conventionally noble gas ions), and go into the gas phase. The atoms or molecules in the gas phase are finally deposited by condensation onto a substrate situated in the vicinity of the sputtering target, where they form a layer.
In sputtering, the problem often occurs that layers of the sputtering material are not deposited homogenously. This problem is also observed with sputtering targets containing indium-gallium-zinc mixed oxides as sputtering material. Thus, during sputtering of these materials small structures, so-called “nodules,” often form that impair the homogeneity of the deposited layer.
In order to solve this problem, European patent publication EP 2 096 188 proposes the use of sputtering targets containing oxides of indium, gallium, and zinc, as well as the compounds ZnGa2O4 and InGaZnO4. To produce these sputtering targets, special powders of indium oxide, gallium oxide, and zinc oxide are mixed, ground to form a powder of raw material, shaped, and finally sintered. With these sputtering targets, the homogeneity of the deposited layers of the sputtering material can in fact be significantly increased.
However, it has been observed that, although at first layers of this sputtering material can be deposited homogenously, this homogeneity decreases continuously as the duration of bombardment, and thus the consumption of the sputtering target increases. Thus, unvarying layer properties cannot be ensured.